The present invention relates to a method of producing a semiconductor device, more particularly, to a method of producing a semiconductor device including a process for forming trenches in a semiconductor substrate and for embedding the trenches using the epitaxial growth method.
In semiconductor devices, such as MOSFETs (insulated gate field effect transistors), IGBTs (insulated gate bipolar transistors), bipolar transistors and diodes, if a region where a drift current flows (hereafter referred to as a drift layer) is made thinner, the current path for the drift current becomes shorter and the ON-resistance becomes lower, but the withstanding voltage is lowered. Conversely, if the drift layer is made thicker, the withstanding voltage becomes higher, but the ON-resistance is raised. As described above, a tradeoff relationship is observed between the ON-resistance and the withstanding voltage in these semiconductor devices.
As a technology for improving this tradeoff relationship, a technology relating to a super-junction structure is known. The super-junction structure is characterized in that the drift layer is not a single semiconductor layer but has s structure wherein an n-type semiconductor region and a p-type semiconductor region, each having a high impurity concentration, are joined alternately and repeatedly (hereafter referred to as a parallel pn structure). As a method of forming the parallel pn structure, a method has been proposed in which trenches are formed in an n-type drift layer by dry etching and the trenches are embedded with an epitaxially grown p-type semiconductor.
As a method of forming trenches in a silicon substrate, the so-called Bosch process is known. In the Bosch process, anisotropic etching is advanced by performing an etching process and a passivation process alternately and repeatedly.
As a method of growing an epitaxial layer having high crystallinity inside the trenches formed by the above-mentioned method, the method described below has been proposed. Trenches are formed in a first conductive type semiconductor substrate. The inner walls of the trenches are washed with dilute fluorinated acid or buffered fluorinated acid and washed with pure water, and then dried. This substrate is placed in a gas furnace, an etching gas containing hydrogen and a carrier gas containing HCl or Cl2 are supplied into the gas furnace, the exposed faces inside the trenches are etched by approximately several nm to 1 μm, and the exposed faces inside the trenches are cleaned. Furthermore, the trenches are formed into a shape opening wider upward by this etching. Following the etching process, annealing is carried out under hydrogen atmosphere, and a growth gas, an etching gas, a doping gas and a carrier gas are supplied into the furnace to allow a second conductive type semiconductor to be epitaxially grown inside the trenches so that the trenches are embedded with the semiconductor (for example, refer to Japanese Patent Application Laid-open Publication No. 2006-019610).
However, when the trenches are embedded with the epitaxially grown p-type semiconductor, if the mask oxide film used for trench forming remains, the following problems occur. FIG. 25 is a sectional view showing an epitaxial layer grown so as to cover the surface of the mask oxide film according to a conventional production method. In addition, FIG. 26 is a sectional view showing an epitaxial layer having embedding defects. As shown in FIG. 25, when the trenches are embedded with a p-type semiconductor 31, the p-type semiconductor 31 protruding above the mask oxide film 23 (hereafter referred to as an over epitaxial layer) is formed so as to cover the surface of the mask oxide film 23. At this time, a stress is generated between the mask oxide film 23 and the p-type semiconductor 31 grown on the surface of the mask oxide film 23 so that crystal defects occur on the surface layer of the p-type semiconductor 31. The occurrence of these crystal defects ranges up to the p-type semiconductor 31 close to the opening portions of the trenches, and the crystal defects remain in the surface layer of the p-type semiconductor 31 after the over epitaxial layer is removed by surface polishing or the like. The crystal defects remaining in the surface layer of the p-type semiconductor 31 become a cause of increase in leak current. Hence, for the purpose of avoiding the influence of the stress caused by the over epitaxial layer, as shown in FIG. 26, embedding is completed before a p-type semiconductor 32 is grown on the surface of the mask oxide film 23 so that the over epitaxial layer is not formed. However, in this case, V-shaped grooves are formed on the surface layer of the p-type semiconductor 32. Even if the surface of the semiconductor device is smoothed by performing a subsequent process, the V-shaped grooves remain on the surface of the p-type semiconductor 32, and embedding defects occur eventually in the p-type semiconductor 32. For this reason, for the purpose of preventing these embedding defects, epitaxial growth is required to be continued until the over epitaxial layer is formed.
Hence, as a method of avoiding the above-mentioned problems, the following method has been proposed in which, after a mask oxide film is removed, trenches are embedded with a p-type semiconductor. More specifically, a mask oxide film for trench etching is formed on the upper face of a silicon substrate, and etching is performed using the mask oxide film to form trenches in the silicon substrate. After the mask oxide film is removed, heat treatment is performed under non-oxidizing or non-nitriding reduced-pressure atmosphere to smooth the inner faces of the trenches in the silicon substrate. Furthermore, an epitaxial film is formed inside the trenches by the epitaxial growth method so that the trenches are embedded with the epitaxial film. Then, the surface of the epitaxial film on the substrate is flattened (for example, refer to Japanese Patent Application Laid-open Publication No. 2002-124474).
Furthermore, as another method, the following method has been proposed. This method is a method of producing a semiconductor substrate in which, after trenches are formed in a silicon substrate, an epitaxial film is formed on the surface of the silicon substrate including the bottom and side faces of the trenches, and the trenches are embedded with the epitaxial film, wherein, at least in the final process of embedding at the time when the trenches are embedded with the epitaxial film, the growth speed of the epitaxial film, which is grown on the side faces of the trenches, at the opening portions of the trenches is made lower than the growth speed at portions deeper than the opening portions of the trenches as a condition for forming the epitaxial film. At that time, an oxide film formed on the silicon substrate is used as a mask when the trenches are formed in the silicon substrate, and the oxide film used as the mask is removed after the trenches are formed and before the epitaxial film is formed (for example, refer to Japanese Patent Application Laid-open Publication No. 2005-317905).
However, the mask oxide film used to form the trenches can be used in subsequent processes to improve the efficiency of the production processes. A method of using the mask oxide film is, for example, as described below. With a configuration in which the mask pattern formed on the mask oxide film is recognized using an exposure apparatus or the like, the mask pattern can be used as a mark for positional alignment between a semiconductor substrate and a photomask, that is, as an alignment marker. Moreover, in a process of smoothing the surface of the semiconductor substrate by CMP (Chemical Mechanical Polishing) or the like, the level of the mask oxide film can be used as the reference level at which the polishing is ended. For these reasons, it is desirable that the mask oxide film should remain unremoved in processes after the formation of the parallel pn structure.
In view of the above, it would be desirable to provide a method of producing a semiconductor device having a super-junction structure, wherein, while a mask used for trench forming remains unremoved, an epitaxial layer having high crystallinity is formed inside the trenches to produce a semiconductor device having excellent device properties.